In a well for piping up oil or gas from an oilfield or gas field, the casing to prevent a collapse of a side wall during/after drilling usually has a nested structure, and multiple casings are nested in the portion near the land surface. In case of the nested casings structure, a big bore corresponding to the outer casing have to be drilled, which leads to high cost. In recent years, in order to solve the problem described above, expandable casing technology, that is expanding the casing in the well. According to this technique, it becomes possible to complete the well by drilling smaller diameter well, compared to the conventional method, leading to the possibility in marked cost down.
However, in case of well construction using one well with uniform diameter from the top to the bottom portion, a considerable large ratio of the pipe expansion is needed, leading to problems such as large bending or perforated portion due to local thinning of the pipe. This has been a hurdle for the practical application of this method. As to the steel pipe with a high expanding performance, the following patents have been disclosed.
Patent Document 1 discloses a seamless steel pipe for an oil well with excellent expandability, which is characterized by a given chemical composition in order to keep the residual austenite phase of more than or equal to 5% volume fraction.
Patent Document 2 discloses a seamless steel pipe for an oil well, which is characterized by a given chemical composition and also by the relationship among the contents of Mn, Cr and Mo and the relationship the contents among C, Si, Mn, Cr and Mo.    [Patent Document 1] JP 2006-9078 A    [Patent Document 2] JP 2005-146414 A